The control of the α-Al2O3 polymorph in industrial scale was achieved in the beginning of the 1990's with commercial products based on U.S. Pat. No. 5,137,774. Later modifications of this patent have been used to deposit α-Al2O3 with preferred coating textures, described in U.S. Pat. No. 5,654,035, U.S. Pat. No. 5,980,988, U.S. Pat. No. 5,863,640, U.S. Pat. No. 6,333,103, U.S. Pat. No. 7,011,867, U.S. Pat. No. 7,094,447, US-A-2006/0199026, and US-A-2006/0141271.
US-A-2007/0104945 relates to a coated cutting tool insert comprising a substrate and a coating to be used in metal machining. The coating contains one or more refractory layers of which at least one layer is α-Al2O3 and contains columnar grains. The layer is characterized by a strong (006) diffraction peak, measured using XRD, and by low intensity of (012), (104), (113), (024), and (116) diffraction peaks.
U.S. Pat. No. 7,201,956 discloses a cutting tool composed of tungsten carbide-based cemented carbide or titanium carbonitride-based cermet, and a hard coating layer provided on the surface thereof, wherein the hard coating layer includes an aluminum oxide layer having an alpha crystal structure, with the highest peak in the inclination section of the (0001) plane of crystal grains within ten degrees relative to the normal of the surface.
Methods to produce binder phase enriched surface zones on cemented carbides containing tungsten carbide (WC), binder phase, and cubic carbide phase are known, e.g. through Tobioka (U.S. Pat. No. 4,277,283), Nemeth (U.S. Pat. No. 4,610,931) and Yohe (U.S. Pat. No. 4,548,786). The patents by Tobioka, Nemeth and Yohe describe methods to accomplish binder phase enrichment in the surface region by dissolution of the cubic carbide phase close to the insert surfaces. Their methods require that the cubic carbide phase contains some nitrogen, since dissolution of cubic carbide phase at the sintering temperature requires a partial pressure of nitrogen, nitrogen activity, within the body being sintered exceeding the partial pressure of nitrogen within the sintering atmosphere. The nitrogen can be added through the furnace atmosphere during the sintering cycle and/or directly through the powder. The dissolution of cubic carbide phase, preferentially in the surface region, results in small volumes that will be filled with binder phase giving the desired binder phase enrichment. As a result, a surface zone consisting of essentially WC and binder phase is obtained. Although the cubic carbide phase is essentially a carbonitride phase, the material is herein referred to as a cemented carbide.
When cemented carbide cutting tools are used in the machining of steels, the tool is worn by different mechanisms, such as abrasive and chemical wear, chipping, and fracturing of the cutting edge. For a coated tool normally having thin surface layers of wear resistant carbide, nitride, carbonitride and/or oxide compounds formed by various vapor deposition techniques, the coating contributes to increase the abrasive wear resistance, but it also acts as a thermal barrier for the diffusion of heat from the cutting surface into the underlying cemented carbide substrate. A high temperature within the edge region in combination with high cutting forces result in an increase of the creep deformation within the affected surface region of the substrate and the cutting edge deforms plastically. Inserts for machining of steel must have good deformation resistance, wear resistance, and toughness.
What is needed is a cutting tool insert with good deformation resistance, wear resistance, and toughness that is useful for machining of iron-based alloys, preferably turning of steel, at fine, medium, rough and interrupted cutting conditions at high cutting speeds. The invention is directed to these, as well as other, important needs.